Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

664 Chapter 12: Intracellular Compartments and Protein Sorting
further translocation across the inner membrane. The remainder of the protein
then crosses the outer membrane through the TOM complex into the intermembrane
space; the signal sequence is cleaved off in the matrix, and the hydrophobic
sequence, released from TIM23, remains anchored in the inner membrane.
In another transport route to the inner membrane or intermembrane space,
the TIM23 complex initially translocates the entire protein into the matrix space
(Figure 12–25B). A matrix signal peptidase then removes the N-terminal signal
sequence, exposing a hydrophobic sequence at the new N-terminus. This signal
sequence guides the protein to the OXA complex, which inserts the protein into
the inner membrane. As mentioned earlier, the OXA complex is primarily used
to insert proteins that are encoded and translated in the mitochondrion into the
inner membrane, and only a few imported proteins use this pathway. Translocators
that are closely related to the OXA complex are found in the plasma membrane
of bacteria and in the thylakoid membrane of chloroplasts, where they
insert membrane proteins by a similar mechanism.
Many proteins that use these pathways to the inner membrane remain
anchored there through their hydrophobic signal sequence (see Figure 12–25A,B).
Others, however, are released into the intermembrane space by a protease that
removes the membrane anchor (Figure 12–25C). Many of these cleaved proteins
remain attached to the outer surface of the inner membrane as peripheral subunits
of protein complexes that also contain transmembrane proteins.
Certain intermembrane-space proteins that contain cysteine motifs are
imported by a yet different route. These proteins form a transient covalent disulfide
bond to the Mia40 protein (Figure 12–25D). The imported proteins are
then released in an oxidized form containing intrachain disulfide bonds. Mia40
becomes reduced in the process, and is then reoxidized by passing electrons to
the electron transport chain in the inner mitochondrial membrane. In this way,
the energy stored in the redox potential in the mitochondrial electron transport
chain is tapped to drive protein import.
Mitochondria are the principal sites of ATP synthesis in the cell, but they also
contain many metabolic enzymes, such as those of the citric acid cycle. Thus, in
addition to proteins, mitochondria must also transport small metabolites across
their membranes. While the outer membrane contains porins, which make the
membrane freely permeable to such small molecules, the inner membrane does
not. Instead, a family of metabolite-specific transporters transfers a vast number
of small molecules across the inner membrane. In yeast cells, these transporters
comprise a family of 35 different proteins, the most abundant of which transport
ATP, ADP, and phosphate. These are multipass transmembrane proteins, which
do not have cleavable signal sequences at their N-termini but instead contain
internal signal sequences. They cross the TOM complex in the outer membrane,
and intermembrane-space chaperones guide them to the TIM22 complex, which
inserts them into the inner membrane by a process that requires the membrane
potential, but not mitochondrial hsp70 or ATP (Figure 12–25E). An energetically
favorable partitioning of the hydrophobic transmembrane regions into the inner
membrane is likely to drive this process.
Two Signal Sequences Direct Proteins to the Thylakoid Membrane
in Chloroplasts
Protein transport into chloroplasts resembles transport into mitochondria.
Both processes occur post-translationally, use separate translocation complexes
in each membrane, require energy, and use amphiphilic N-terminal signal
sequences that are removed after use. With the exception of some of the chaperone
molecules, however, the protein components that form the translocation
complexes differ. Moreover, whereas mitochondria harness the electrochemical
H + gradient across their inner membrane to drive transport, chloroplasts, which
have an electrochemical H + gradient across their thylakoid membrane but not
their inner membrane, use GTP and ATP hydrolysis to power import across their